Microchip lasers and miniature lasers that uses laser microchip's are small, robust, compact, diode pumped solid state lasers which can be manufactured in large quantities at a low cost. For a general technology background and examples on aspects, advantages and interesting application areas of microchip lasers reference is given to the article "Microchip Lasers and Laser Arrays: Technology and Applications", Optics and Photonics News, November 1995, pp. 16-19. Examples on interesting application areas for microchip lasers include transmission of cable TV signals, image generation, material treatment, etc.
The microchip laser concept was developed at MIT Lincoln Laboratory in the middle of the 1980s and aims to use semiconductor packaging and fabrication technology for manufacturing of lasers. Simple microchip lasers are made form a rod of laser material, normally a crystal or a glass, such as Nd:YAG. The rod is cut into wafers which are polished to show planar and parallel surfaces which is then coated with dielectric mirrors, usually by some kind of vacuum deposition technology. The wafers are then cut into chips using conventional semiconductor cutting technology. Laser crystal chips usually have an area of approximately 1.times.1 to 4.times.4 mm.sup.2 and a thickness in the radiation direction of less than 4 mm. The microchip laser technology thus provides for mass production of laser microchips. For example, more than 2,000 chips can be fabricated from a 1 inch long Nd:YAG rod.
Furthermore, different types of functional elements, such as Q-switches or frequency doubling elements etc., can be integrated in the laser cavity. By using pick and place technology, the laser chip and one or more functional elements may be brought together and contacted through thermal diffusion bonding or similar. When the different elements are mounted together in this way, and mirrors are applied directly on the chips/elements, the appearance of air gaps is eliminated in the laser cavity, which eliminates risk for reflections which may disturb the lasing. However, laser microchips can also be used in so called miniature lasers, in which the laser microchip and other incorporated functional elements not necessarily are arranged in contact with each other but instead are separated from each other by small gaps.
Just as for conventional solid state lasers, the laser material is doped with ions of a rare earth metal that provides the lasing. As the thickness of the laser chip is small compared to other types of solid state lasers, the laser radiation can only be built up over a relatively short distance. To compensate for this limitation, the laser microchip material is given a high degree of dopant as compared to other solid state lasers. In a micro-hip laser, the doping provides for more than 0.1 percent by weight of the laser material, while the degree of dopant in a conventional solid states laser usually is far below 1,000 ppm, for example 50 ppm for a 10 cm long laser crystal. The high degree of dopant in the laser microchip provides a short absorption length, which among other things makes it possible to store a large amount of energy in a small volume, but also leads to a shorter lifetime for electrons excited to the meta- stable upper laser level.
The most common and important dopant for laser microchips is neodymium (Nd), but erbium (Er), prasodymium (Pr), holmium (Ho), and ytterbium (Yb) are other examples on important rare earth metals that can be used as dopants. Ytterbium is of special interest, as it can ease over a broad spectrum or wavelengths.
Several different host materials may be used. YAG--Yttrium Aluminium Garnet (Y.sub.3 Al.sub.5 O.sub.12) and yttrium vanadate (YVO.sub.4) are two of the most commonly used crystals today. YAG has a comparatively long lifetime for the meta-stable state, but a lower absorption cross-section (i.e. a lower absorption capacity) than yttrium vanadate, which enables the latter to be made shorter. Many other crystals may be used, such as YLF (yttrium-lithium-flouride), LNP, LSB, SVAP, and GdVO.sub.4 (gadolinium vanadate). Even glasses and plastics can be used as host materials.
Most Nd lasers are used at the strong transition .sup.4 F.sub.3/2.fwdarw..sup.4 I.sub.11/2. This provides lasing at wavelengths slightly longer than 1 .mu.m, somewhat varying as dependent on the host material. For Nd:YAG this wavelength usually appears at 1064 nm. Other interesting transitions that may be used are .sup.4 F.sub.3/2.fwdarw..sup.4 I.sub.9/2, which gives lasing at shorter wavelengths (946 nm for Nd:YAG), and .sup.4 F.sub.3/2.fwdarw..sup.4 I.sub.13/2, which gives lasing at a longer wavelength (1360 nm for Nd:YAG).
The laser microchip is usually pumped by a diode laser that gives a wavelength snatched to the absorption band of the used rare earth metal. The pumped laser may be single mode or multimode. The function of the diode laser is only to excite electrons in a small volume of the laser microchip and thereby transfer the largest possible amount of energy to the mode of the laser micro-chip. A relatively cheap diode laser is thus preferably used, as the laser microchip will transfer the relatively poor spectral and spatial properties of the diode laser output into a laser beam having pure spectral and spatial properties and low noise. As a result of the high doping and short length of the laser microchip (and the corresponding short laser cavity which gives a large mode spacing), microchip lasers are characterized by a tendency to lase in single mode, which is desired in many applications.
A very important property of materials such as YAG is that dn/dT (the change in refractive index with temperature) is positive, which means that a thermally induced lens is created by the heat generated by that part of the pump energy which is not utilized for the lasing in the microchip laser. The thermal expansion of the material also contributes to the creation of this lens. The lens stabilizes the laser cavity, which would otherwise have been an unstable (coplanar) cavity. For a chip having plane, parallel mirror surfaces, a stable resonator may be formed if the pump light is concentrated This phenomenon gives an automatically stabilized resonator, which enables simple manufacturing and alignment.
A very important type of microchip lasers and miniature lasers Which utilize laser micro chips are such that 25 uses so celled intra-cavity frequency doubling. These types of lasers comprise an intra-cavity arranged functional element in the form of a frequency doubling crystal that provides frequency doubling (SHG--second harmonic generation) of the fundamental light wave, i.e. which converts the laser radiation of the fundamental wavelength from the laser microchip into laser radiation of half the fundamental wavelength. Examples on such intra-cavity frequency doubling in a microchip laser has been described in the international patent application WO 90/09688.
Intra-cavity frequency doubled microchip lasers can show a very high conversion efficiency even at low pump power. This is caused by the laser mode being concentrated i.e. has a high intensity, in the short microchip laser cavity, and by the laser mirrors being made high reflecting at the laser wavelength, which means that the fundamental light wave is confined between the mirrors to build up a high intensity.
During frequency doubling, two coherent photons at the fundamental wavelength, each having the energy {character pullout}.omega. induce a polarization at the double frequency in the non-linear material. The induced polarization is a source term for emission of a new photon having the energy 2{character pullout}.omega.. To achieved an efficient frequency doubling, phase matching is required, which means that the polarization induced at different places in the material emit frequency doubled radiation coherently. This require that the two waves .omega. and 2.omega. perceive the same refractive indices. This is normally not possible, as the dispersion makes the refractive index at the shorter wavelength larger than the refractive index at the longer wavelength.
When using intra-cavity frequency doubling in microchip lasers, so-called birefringent phase matching is utilized. Here, a difference in refractive index for different polarization's in uniaxial and biaxial non-linear crystals is utilized. The difference in refractive index makes it possible to achieve the desired phase matching.
Birefringent phase matching is used since birefringent crystal chips may be mass produced in a similar way as the laser chips mentioned above. For example, a KTP crystal rod can be cut into wafers along those planes that are required to provide a crystal having the necessary double refraction for birefringent phase matching in different directions. These wafers can then be cut into chips that can be bonded to the laser chips by pick and place technology, as discussed above.